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Efficient Landing


Duckytrask
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What is considered the most efficient way to land a spacecraft on a body without an atmosphere.

I can't remember where but i heard that you first cancel all your horizontal velocity, and then suicide burn at the very end.

Edited by Duckytrask
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Yes it's suicide burn.

And I think there's no need to cancel horizontal velocity, just burn retrograde until your trajectory touch the ground where you want to land and full thrust to cancel all velocity right before touching the ground.

EDIT: not sure at all about that

It will require several tries...

Edited by Gaarst
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There are two ways to do it efficiently. The suicide burn is efficient (if difficult). Lowering your periapse as much as possible over the selected landing area and cancelling horizontal velocity just over the landing site is also efficient. Probably most people use a little of both. Keep periapse high enough to avoid terrain, and use suicide burn once horizontal velocity has been mostly killed.

That being said, "efficient" doesn't always mean "best". Best is usually a compromise between brutal efficiency and ease of implementation. Suicide burns are called that for a reason... TWR changes when burning fuel, so calculating a suicide burn accurately requires integration, not something most people are willing to do while plummeting toward the lithosphere... but for small burns at low velocities/altitudes, a guesstimate is usually good enough and errors are kept small so they don't affect the overall mission very much.

I always come in low and fast, and kill horizontal velocity a bit at a time... just enough to keep from overshooting my target. I don't flip vertical until maybe 100-200m altitude (when my shadow becomes "solid" and doesn't flicker).

- - - Updated - - -

Yes it's suicide burn.

And I think there's no need to cancel horizontal velocity, just burn retrograde until your trajectory touch the ground where you want to land and full thrust to cancel all velocity right before touching the ground.

It will require several tries...

That's not the only efficient way to land. There are others that are equally efficient from a math standpoint. You fail to consider whether a method can truly be considered efficient if the implementation is so difficult that a person cannot do it effectively. If you are flying your rockets by hand, trying to perform a "suicide burn" and end up having to let yourself down gently so you don't go crashing into the surface, you've just turned an "efficient" method into the worst possible way to land.

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For that matter, the constant altitude method of landing is actually more fuel efficient than a suicide burn in most cases, but not by a large amount unless your lander has a low TWR. As your TWR increases, the two methods converge. The suicide burn is the most efficient manner of landing in the case where you're already on a not-easily-fixed suborbital trajectory.

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If you're seriously considering a suicide burn Kerbal Engineer Redux has a optional suicide burn readout.

I don't really understand how to use KER's suicide burn readout, could you explain?

I've always done suicide burns with enough breathing room to go down gently afterwards, which is inefficient but pretty much every lander I use is different (career mode, science progress, experimentation) so it's tough to get used to it.

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KER's suicide burn info is available but must be installed in an existing display or a new HUD you create. It will show suicide burn altitude, burn duration, burn dV, and time until beginning suicide burn. Time til burn is the one you're really most concerned with as it tells you when to press "z". I still don't trust it 100%, but I've used it without ill effect (but my velocity wasn't that high either... I basically used it in conjunction with my method of cancelling horizontal velocity just above landing area).

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I don't really understand how to use KER's suicide burn readout, could you explain?

I usually just go by the "Suicide Burn dist" readout; I try to hit Z as close to zero (but still above it) as possible. Hasn't failed me yet :). If that number drops below zero you're pretty much toast, short of a miracle :P

I still don't trust it 100%, but I've used it without ill effect (but my velocity wasn't that high either

I'm in the same boat, although I have used it for fairly aggressive mun landings (in excessive of 400m/s) without issue. I have heard of it being unreliable though, but then again that could easily be PEBKAC issues :D

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then again that could easily be PEBKAC issues :D

If I had to guess... I'd say the problem is integrating a complicated ever-changing equation in a very short amount of time. Often only analytical solutions yield the exact answer. Sometimes formulae can be used to get a good approximation, but it's likely that one doesn't exist for the various situations in KSP and that instead the only solution is to find a good infinite-sum representation and then add an arbitrary number of terms... though even in that case there are situations where it can be much less accurate. I imagine it is based off of terrain altitude at the predicted landing site (or directly under the ship... which is even worse)... in which case as the terrain changes the estimate will change depending on slope. This makes sense if you think about it, considering that KER can't possibly keep track of everything all the time, it operates procedurally with limitations on tick time. With changes in terrain and such... even integration would prove less than reliable in some situations.

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Most efficient is a gravity turn (remember the Oberth Effect). However it is even more complex to calculate then a vertical suicide burn due to changing terrain features of the "impact" site.

I use the KER suicide burn info as well. The only needed entry is time till burn. Altitude or Duration can be used for context to reckon timing accuracy. It worked acceptably for my .90 minimus Ion lander.

Procedure:

> (Optional) Enter low orbit (to minimize per. velocity)

> Burn to sub-orbital trajectory that intersects the surface 30s-1m after your desired landing zone

> Set navball to retrograde tracking if possible. This frees you to focus on site, thrust, and altitude.

> Engage engines to 100% BEFORE the suicide counter hits 0. Closer is more optimal, but unless you have a hotkey for more power, you don't terribly want to chance overshooting it for a few m/s.

> You were likely early on starting the burn. Adjust thrust to zero time til burn timer. Lowering thrust will cause it to decrease. If you overshoot, you can always raise throttle again. The less thrust you use high, the more efficient.

> At some point here you should evaluate the landing zone. If it's not flat, you may need to take manual control to avoid a RUD. Point your engine towards the features you want to avoid and have fun cowboy!

> Near touchdown, you may want to lower your throttle. A TWR higher than .6-.8 (dependant on local gravity) and you will bounce on landing!

> Cut engines at/right before touchdown.

> New SAS fixes this, but you also used to need to cut SAS on landing as well if you followed retrograde. It should now revert to stability control on landing.

If you want more efficiency, space planes can lithobrake (and survive) if they have suitable vertical translation thrust! Similar to above, but you direct thrust on the vertical vector only to control vertical velocity and let your brakes cancel horizontal velocity after you land. For obvious reasons, this technique has even more limitations on landing site suitability. Also, this is most efficient when you touch down at near ORBITAL speeds!

Edited by ajburges
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impyre: sure, it's a complicated problem, although given this is a suicide burn we're talking about if you're going to leave it to the last second you had better be pointing retrograde with full throttle. That makes it possible, in principle, to predict your overall trajectory pretty accurately, assuming you don't screw up too badly during the burn. I think the main inaccuracies are when the ship isn't pointing perfectly retrograde the entire time; in other words, pilot error. As for the computational cost, I can't imagine this is anywhere near as complex a problem as projecting your path through an atmosphere in real time; the trajectories mod managed that just fine even on my old computer back in 0.90 with FAR installed.

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Most efficient is a gravity turn
This. Somewhere in the dusty archives, there was a challenge thread that came to the same conclusion. Sure wish I could find it now. Well, the basic idea was preserve that horizontal velocity because hovering uses fuel. Lower your orbit so Pe is just beyond your landing site, then follow retrograde and suicide burn. Just like the Goddard Dilemma for ascent, but reversed.

The Apollo landings did basically the same thing, but with TWR much lower than any of us have patience for!P04_F03_625.jpg

P04_F04_625.jpg

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I think I do the gravity turn thing, not really sure. I burn slowly kinda half way between Pe and vertical to lose velocity while keeping my landing position still (this has the effect of lowering the Ap and bringing it back to your ship even though you already passed it). When I get really close I kill the lateral velocity and come down using a slow burn to maintain speed, but at that point I'm under 100 m/s. I'm not sure how efficient that is or if it is the "best" way, but it is easy for me and it's certainly more efficient than killing lateral velocity at 20km like I used to do when I first started the game.

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Lower your orbit so Pe is just beyond your landing site, then follow retrograde and suicide burn. Just like the Goddard Dilemma for ascent, but reversed.

The Goddard Dilemma only applies when there is an atmosphere, and is only used for ascents. The whole point of the gravity turn is to keep the rocket pointed into the relative wind.

A Mun lander I flew about 2 years ago started the landing burn with a TWR of 1.09 relative to Mun. I heard from someone that doing a gravity turn landing (only pointing at retrograde) required the craft to start at 30km altitude. It also used a lot more fuel than coming in low and killing horizontal velocity. This is very apparent in low TWR craft. That said, you can get away with any sort of landing method if you have a high TWR.

Here's the video before someone else posts it:

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Thats a good method for landing without wasting too much fuel, but make no mistake, it is not the most efficient - there are significant cosine losses.

Most efficient is a perapsis right above the ground, and a suicide burn.

-Do not assume a suicide burn is a vertical drop, in this case, its like a suicide reverse gravity turn.

A suicide burn will kill you if you start too late, but if you have a very low perapsis, you shouldn't waste too much descending if you start too early.

That method shown is very tolerant of timing your burn, but you will use more dV than a well executed low perapsis suicide burn.

Is probably safer and easier than a suicide burn, while still being reasonably efficient.

FWIW, I just set a low perapsis, do a retroburn until my trajectory intersects the ground (and maybe maneuvers to move the intersection to where I actually want to land), then place a maneuver node at the intersection and drag retrograde as far as it will go.

If it tells me its a 50 second burn, I start the burn 25 seconds to the maneuver node.

Assuming a constant accelration, my average speed will be about half the speed its using to calculate the time until the maneuver node.

As I will lose mass while burning fuel, acceleration will increase, and it gives me a nice margin for error.

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Here's the video before someone else posts it:

This is a pretty cool approach, it's a much more challenging method for the pilot and isn't very practical on more broken terrain (see: practically every normal body in KSP). At least, seems that way to me. I've tried if a few times, but never really managed to make it work out well. Maybe it'd be easier with Mechjeb and such, I dunno.
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KER's suicide burn info is available but must be installed in an existing display or a new HUD you create. It will show suicide burn altitude, burn duration, burn dV, and time until beginning suicide burn. Time til burn is the one you're really most concerned with

I was trying to do this, but when editing an existing display or building a new one, I can only find Suicide Burn Altitude, Distance, and dV(In the Vessel section) - no time to burn or duration of burn. Where do you find those? I was hoping to add another HUD display with them, but I can't seem to find them.

Also, what exactly do Altitude and Distance mean in this context? Altitude is the Altitude at which you should start the burn? I've noticed that Altitude keeps decreasing as my lander falls, so I assume it's NOT the point where you should start the burn (as I fall, speed increases, so I'd need to start the burn higher over time, no?) Is that over ground, or over sea level? What does distance mean?

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I was trying to do this, but when editing an existing display or building a new one, I can only find Suicide Burn Altitude, Distance, and dV(In the Vessel section) - no time to burn or duration of burn.

That seems consistent with my copy of KER.

Also, what exactly do Altitude and Distance mean in this context? Altitude is the Altitude at which you should start the burn?

Essentially yes, but I think it's the altitude above terrain. That might explain why it keeps changing, although I'm not sure. Assuming the time to burn is not a thing that's somehow just missing for both of us, the distance is the one you care about. hit Z when it's close to but above 0 and you *should* land safely, provided you're pointing retrograde while burning.

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The Goddard Dilemma only applies when there is an atmosphere, and is only used for ascents. The whole point of the gravity turn is to keep the rocket pointed into the relative wind.

A Mun lander I flew about 2 years ago started the landing burn with a TWR of 1.09 relative to Mun. I heard from someone that doing a gravity turn landing (only pointing at retrograde) required the craft to start at 30km altitude. It also used a lot more fuel than coming in low and killing horizontal velocity. This is very apparent in low TWR craft. That said, you can get away with any sort of landing method if you have a high TWR.

Here's the video before someone else posts it:

That is all fine if you don't mind where you land. But if your building a surface base, flying that low is going to be hard to predict where you will land. I do basically the same landing but from a bit higher up and I come down at a steeper angle so I can control from map mode where I will end up.

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Thats a good method for landing without wasting too much fuel, but make no mistake, it is not the most efficient - there are significant cosine losses.

If we just look at the deltaV it requires to go from orbital velocity to a complete stop then the standard gravity turn is the least efficient, while maintaining altitude and constant angle of attack tend to end up with similar deltaV losses. My intuition would tell me that constant angle of attack should be the most efficient if performed perfectly, but it's hard to get it to end up with no vertical component at the end, so it often ends up getting slightly higher losses. Anyway, the retrograde gravity turn is consistently less efficient in my tests.

You might think the gravity turn should get less deltaV losses, but actually that maneuver trades kinetic and potential energy very inefficiently. It traverses the most altitude when it is moving the slowest, where increase in kinetic energy will result in the most change in velocity.

The other two traverse the most altitude while are moving as fast as possible. While they do get cosine losses it is used to fight gravity, so I think they simply become the gravity losses.

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Le LM has to "hover" because it had a low TWR. It's the same deal with Tylo. You can land with brute force and high TWR with a suicide burn at a cheap DV (the higher the TWR, cheaper it is in delta-V [not in fuel...]. But if you TWR is too low, you'll simply crash.

Killing (reducing) horizontal first while keeping vertical under control is doable with low TWR ships, until the TWR goes higher and you can counteract vertical force.

This manoeuver has a delta-V price. But the ship (SSTO) I designed for Tylo wasn't able to do the suicid burn. I did the "hovering" manoeuver easily and take off for 6000m/s.

PS : in beta 0.9.

Now, only math can confirm it.

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A suicide burn with TWRs below 3 costs about 33% of your dV because gravity is 1. With a retrograde gravity turn (assuming you pitch up bit by bit in order to negate vertical velocity) and a TWR of 1.5 assuming you go for the final vertical descent at 200m has gravity losses of about 15-20%, even though the TWR is low because initially you suffer no losses to gravity and you only really begin to suffer gravity losses later on when your horizontal velocity is lower. This is simply because apparent gravity (that is, actual gravity minus the centripetal acceleration from your horizontal velocity) only begins to exist when you begin to reduce your orbital velocity, apparent gravity is more important than normal gravity for as far as horizontal descents are concerned.

High TWRs themselves also come with extra costs, because more TWR means that you have higher engine mass and thus less dV or higher launch costs. The savings vs costs of TWR begin to converge at TWRs of about 1.5.

Naturally, initial TWR (when you begin the burn) is less than final TWR and this needs to be kept in mind when designing vessels with landing costs in mind.

Pages 8 and 9 of this NASA document may be particularly desireable to read: https://www.hq.nasa.gov/alsj/nasa58040.pdf.

Edited by SanderB
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(Edit: Didn't see that there was a second page to this thread where this has been covered)

Basically I'm going to say essentially the same thing everyone else has said, but I found moon landing descents to be a good reference for the process.

Some of the diagrams are not scaled well, but basically you want to drop your periapsis to be very close to the surface. You don't necessarily have to cancel ALL horizontal movement before flipping to a vertical descent. This apollo landing video shows this flipping up at about 1:30:

After you flip up, as you descend vertically at an angle, you will be pointed up, but angled towards the retrograde marker. You will continue to burn, but not at full throttle. Since you are angled slightly towards retrograde, then you will be losing horizontal velocity, and therefore can continue to turn closer and closer to vertical.

Here's the rule of thumb when descending at an angle: The closer your retrograde marker is to vertical, the less horizontal velocity you have. To reduce horizontal velocity, then angle yourself slightly towards retrograde. In other words almost pointing straight up, but slightly eskew towards the retrograde marker. If you need to straighten up quickly, then you can angle yourself past the retrograde marker and burn.

Just like curving your ascent, an efficient descent would usually be curved. On a body with little gravity and no atmosphere, that's going to be a very low curve.

This diagram shows the orbiter above, and the lander below, moving right to left. "Throttle maximum thrust" is where you are cancelling out much of your horizontal velocity.

powered-descent.jpg

If you look very closely at the bottom left of this diagram, you'll see how much of the curving is at the very end of the descent just after the "7" marker.

approach-phase.jpg

Not everything about the way moon landers maneuvered was about efficiency. My understanding is the final leg required visual guidance to ensure they hit an ideal landing spot and avoided some dangerous landscape, so they wanted to be flipped up to allow them to have a good visual.

It would be really cool if they added ability to have audible velocity or altitude measurements, so that you could focus on other aspects of piloting. Perhaps only available when a second kerbal is on board :)

Edited by AaronLS
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Woah,

Ignoring human error, anything less than full throttle is typically wrong in a vacuum (except for burns that specifically rely on the integration). Less thrust correlates to longer burns. Longer landing burns leads to heavier gravity loss in landing and takeoff. The reason we control throttle in atmo is to balance gravity loss against atmospheric loss. If there's not atmo loss, there's nothing to balance.

Apollo model landings are good, but they are not the most efficient (still pretty good). As mentioned, they needed visual observation of the landing site for final approach.

There are a few effects I am aware of that are notable for landing fuel efficiency. Oberth and force of gravity. Theoretically, the most efficient landing considering those two would be to arrange a (near) surface level periapsis from initial (highest) orbit. Then suicide burn centered at periapsis (the greater velocity from the elliptic orbit should realize more kinetic energy via the Oberth effect than lowering the orbit before the maneuver). This approach is not practical for most though. The more extreme circumstances would surely lead to efficiency losses from execution errors that would eliminate any gains! Not to mention TWR has a bigger impact on efficiency in this maneuver.

Still, if we are going to debate efficiencies, why is controlled lithobraking not getting more attention! Depending on object mass and rotational speed, you can get several hundred dV without any fuel usage. More precise landings don't need any burns after the deorbit burn! Plus those Kerbals love the ride.

Edited by ajburges
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